Conventional color photographic images are formed via a chromogenic development process. After exposure of a color photographic element, the object scene is stored as a composite of red, green and blue silver halide latent images. During the color development process, these silver halide latent images are reductively developed, and an oxidation product of this development reacts with cyan, magenta and yellow dye-forming couplers to provide the desired subtractive primary color images. The composite dye image is then formed by the superpositioning of the cyan, magenta and yellow dye images to afford a reproduction of the original scene. The controlled conversion of silver halide latent image to color dye image is the goal of color photographic chemistry. The yield of dye color density from each unit of silver halide developed is a measure of coupler activity. The higher the activity of a coupler the less silver halide is needed to allow effective image formation. The reduction in the amount of silver halide used in a photographic systems can lead to improved photographic image reproduction, lower cost photographic products, and less potential environmental damage from development processes. Coupler activity, as defined herein, is composed of two prime factors: (1) the efficiency of the dye formation process, i.e., the chemistry converting coupler to dye, and (2) the light absorption properties of the chromogenically formed dye, i.e., the dye's spectral bandshape and extinction. Improvements in dye extinction can lead to desirable improvements in coupler activity and thereby to silver halide reductions.
Two further important features of photographic reproductions are their color fidelity and their image stabilities. To efficiently reproduce a wide gamut of hues, the dyes comprising the color image must exhibit relatively sharp cutting spectral curves. Additionally, the dyes' spectral response curves must be carefully placed, i.e., have a well positioned maximum absorption and curve shape, to afford the best possible color reproduction.
Color photographic images slowly degrade when stored under ambient conditions. Pictures held in the dark, that is, stored in albums, boxes or slide trays and not exposed to direct light, degrade primarily via hydrolytic mechanisms. Images exposed to light fade both via the hydrolytic mechanisms as well as via photochemical processes. The stability of a color image is clearly dependent upon the stabilities of its component dyes. It is apparent from these considerations that the hydrolytic stability of photographic dyes is of primary importance to image stability. The destruction of photographic dyes may be catalyzed by either acids or bases; dyes that are robustly stable to various hydrolytic conditions will provide more stable photographic images.
Various efforts have been undertaken to provide a yellow dye-forming coupler that exhibits an improved combination of activity and image stability. A new class of photographic yellow couplers was recently patented by Bernard Clark, et al. in U.S. Pat. No. 5,674,667. This patent discloses a novel class of pyrroloyl-acetanilide couplers that exhibit a combination of improved dye contrast and dye stability. These characteristics were found to be comparable or superior to the couplers used in commercial photographic products. Comparative examples of this art are found hereinafter.
In spite of past efforts, there is still a need for improved yellow couplers, especially those that provide dyes of improved stability.